Acoustic windows such as sonar domes for use in transmitting or receiving acoustic wave form signals in a liquid environment are known. Traditionally, these windows have consisted of a single thickness of a metal such as steel that may optionally have been covered by a biologically active substance such as a rubber containing a biocide, in order that biofouling of surfaces of the window may be inhibited.
Typically such windows on an exterior surface have interfaced with a body of free liquid such as an ocean, lake or tank. Such windows, on the interior surface, traditionally have at least partially defined a chamber filled with water or another liquid. Substantial efforts have been expended to configure such windows to be acoustically "clear", that is producing a desirably low distortion and attenuation of sound wave energy being passed through the windows and, equally, a desirably low distortion of the angle characterizing the impingement of the wave energy against the window.
Such windows have been subject to certain undesirable characteristics. For example, windows made of a rigid material such as steel can generate significant quantities of acoustic noise associated with the passage of water over the window and can transmit significant quantities of acoustic noise arising from vibrational frequencies associated with the operation of machinery aboard a ship upon which a window is embodied. In addition, these relatively rigid windows can generate a significant bounce or reflection effect for acoustic wave form energy impinging upon the window surface. Such bounce can result in a substantial reduction of signals being transmitted through the window, and where reflection occurs from interior surfaces of the window during transmission of an acoustic wave form from within the chamber defined by the window, spurious or erroneous determinations of and/or making of an echo can result.
It has been suggested that alternate materials to steel or other metals be employed in the fabrication of domes. Fiber reinforced plastics (FRP) have been suggested as a suitable window material, such FRP materials have demonstrated enhanced corrosion resistance over steel but have generally been subject to many of the same difficulties characterizing steel with respect to acoustic clarity, reduction, and reflective characteristics.
Windows such as sonar domes can be required to transmit acoustic energy having a frequency ranging from about 500 hz to about 500 khz. These frequencies correspond to wavelengths of about 3 meters to about 0.003 meters in water, with the wavelengths being subject to some variation depending upon the material through which the wave form is being propagated. With traditional domes of metal or reinforced plastic, where the thickness of the material from which the dome is fabricated deviates substantially from a 1/2 wave length of the acoustical frequency being transmitted through the dome, reduction such as through insertion loss, that is 20 log P.sub.o /P.sub.t where P.sub.o is the incident pressure of the wave and P.sub.t is the transmitted pressure, can become unacceptable. A sonar dome structurally must be built to withstand a particular structural loading. This construction results in an inherent thickness in the material of construction. Where this thickness substantially deviates from 1/2 the wavelength being transmitted an effective blindness to certain acoustic waveform frequencies can result by simple reduction of the waveform energy transmitting across the material thickness.
Naturally sonar domes are not the sole use for acoustically transparent materials; frequently it is desired that acoustic waveform energy be transmitted through a window or covered aperture in a vessel hull. The same constraints that affect performance of conventional sonar domes also may affect the acoustic performance of such windows.
Structural configurations in forming sonar domes and windows have traditionally focused material selection considerations upon elevated modulus materials, that is materials having a Young's modulus of in excess of at least about 100,000 psi (6895.times.10.sup.5 kPa) and more frequently in excess of about 1,000,000 psi (6895.times.10.sup.6 kPa). These materials generally are possessed of an elongation break characteristic approaching zero and a sound propagation velocity characteristic too elevated to be used in a desirable, thin tunable window, and the use of such rigid, high strength materials has tended to make "tuning" sonar domes and windows formed with such materials quite difficult. The properties of materials of construction for the sonar domes or windows taken together with the structural loading imposed upon such domes and windows has tended to establish the acoustic properties of the sonar dome without much residual flexibility for tuning of the properties such as clarity, reduction and the like.
A sonar dome or window, tunable to substantially reduce reduction of sound wave frequencies, upon passage through the sonar dome or window, could find substantial application in both the military and commercial areas. Equally, a dome or a window formed of one or more materials configured to reduce the reflective signals during passage of an acoustic waveform signal therethrough could find substantial utility.
Likewise, a construction for sonar domes and windows wherein the sonar dome or window is possessed of elevated self damping properties, could find substantial utility in reducing noise and spurious signals resulting from vibrations engendered as examples, by the passage of water along the sonar dome or window, or by the transmitted vibration of machinery and equipment aboard the vessel embodying the sonar dome or window.